Remove obsolete import of //build/config/chromeos/ui_mode.gni

The file is about to be deleted.

Bug: b:354842935
Change-Id: I166a8ce7d5f6166e6e46dfeb1f366e0247b93bf7
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/475900
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Auto-Submit: Georg Neis <neis@google.com>
Cr-Commit-Position: refs/heads/main@{#47814}

build_overrides/partition_alloc.gni

diff --git a/build_overrides/partition_alloc.gni b/build_overrides/partition_alloc.gni
index 3f674f8..99fc3aa 100644
--- a/build_overrides/partition_alloc.gni
+++ b/build_overrides/partition_alloc.gni
@@ -9,7 +9,6 @@
 # Dependencies from //build defining variables used by partition_alloc.
 import("//build/config/android/config.gni")
 import("//build/config/cast.gni")
-import("//build/config/chromeos/ui_mode.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/cronet/config.gni")
 import("//build/config/dcheck_always_on.gni")